charge and current

• • Electric current is defined as the rate of flow of positive charge carriers
  • It is measured in units of amperes (A) or amps
  • The symbol for current is I
• • Charge is a property certain particles have. It can either be:
  • A positive charge (+) (eg. proton)
  • A negative charge (–) (eg. electron)
  • A neutral (no) charge (eg. neutron)
• • An atom is neutral. This is because it has an equal number of protons (positive charge) and electrons (negative) charge
  • However, just the nucleus which is made up of protons and neutrons is positively charged
• The Coulomb

  • The unit of charge is the Coulomb (C)
  • This is defined as the quantity of charge that passes a fixed point per second when a current of 1 A is flowing
  • The coulomb (C), in SI base units, is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e. 1 C = 1 A s

   
• • The charge on charge carriers is quantised
  • This means the charge comes in definite, finite quantities
  • In this way, the quantity of charge can be quantised depending on how many protons or electrons are present
  • Positive and negative charge has a definite minimum magnitude and comes in multiples of that magnitude
  • This magnitude is the elementary charge, e = 1.60 × 10-19 C
  • The magnitude of the charge just refers to its value, rather than whether it is positive or negative
• • Electric current is the rate of flow of charge carriers
  • This is defined as the movement of
  • Electrons in metals
  • Ions in electrolytes
• In metals  the flow of charge is made up of electrons

  • The metal ions are closely packed and arranged in a crystal lattice structure
  • The atoms have many free (delocalised) electrons that are free to move randomly
  • These are sometimes known as conduction electrons
  • These are what makes metals good conductors of electricity
• • ons are atoms that have lost or gained an electron. This means they are either:
  • An anion - a negative ion (gained an electron)
  • A cation - a positive ion (lost an electron)
  • An electrolyte is a substance that produces an electrically conducting solution
  • The charge carriers are not electrons, but cations and anions
• • In electrical wires, the current is a flow of electrons
  • Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal
  • Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
  • This is the opposite to the direction of electron flow, as conventional current was described before electric current was really understood
• • Kirchhoff’s first law states that:

  The sum of the currents entering a junction always equal the sum of the currents out of the junction
• • Kirchoff's First Law
  • This is a consequence of conservation of charge
  • Charges cannot be created nor destroyed
  • Therefore, the current shouldn’t decrease or increase in a circuit when it splits
• • In a series circuit, the current is the same at any point
  • In a parallel circuit, the current divides at the junctions and each branch has a different value
  • Kirchhoff’s first law applies at each junction
• The drift velocity is the average velocity of the charge carriers travelling through the conductor
• I = Anev

  • The number density n represents the number of free charge carriers (electrons) per unit volume
  • Conductors, such as metals, have a high value of n
  • Insulators, such as plastics, have a small value of n
  • This equation shows:
  • v is inversely proportional to n meaning more charge carriers per unit volume will slow down their speed through the conductor
  • I is directly proportional to n since greater n means greater charge is flowing and therefore a larger current I. When the value of n is lower, the charge carriers must travel faster to carry the same current
• Conductors have a very large value of n

  • This means there are many free electrons per unit volume moving through the material
  • Since current is the rate of flow of electrons, this makes them very good electrical conductors
• Semiconductors

  Materials that have a conductivity between conductors and insulators
• Semiconductor conductivity

  Depends on their temperature
• At low temperatures

  Resistivity rises, therefore conductivity falls
• At high temperatures


  Resistivity falls, therefore conductivity rises
• Metal conductors


  Behave in the opposite way
• Semiconductors


  • Have very intermediate conduction properties
  • Neither very good nor very poor in terms of their conducting properties
• Doping


  When impurities are added to semiconductor materials to help improve their conducting properties, by increasing the value of n
• • Insulators have a very small value of n
  • This is close to 0
  • This means there are next to no free electrons per unit volume moving through the material
  • With no current, this makes them very poor electrical conductors